1. Field of the Invention
The invention relates to methods and apparatus for the extrusion of tubular, thermoplastic resin films.
2. Brief Description of the Prior Art
The prior art literature is replete with descriptions of methods and apparatus for extruding tubular films of thermoplastic resins. Representative of such descriptions are those found in the U.S. Pat. Nos. 3,7754,067; 4,479,766; 4,626,397; and the Japanese Pat. No. 84-058454/10.
The U.S. Pat. No. 3,754,067 (D.R. St. Eve et al.) issued Aug. 21, 1973 describes tubular film extrusion apparatus which includes a first cooling means for cooling the molten extrudate to a temperature above the crystalline formation temperature. A second cooling means follows annealing to solidify the film.
The apparatus described in the U.S. Pat. No. 4,479,766 (Planeta) issued Oct. 30, 1984 is relatively sophisticated, employing a second cooling means or air ring which may be moved along the length of the extruded tube (below the point of tube expansion) for obtaining a stable tube.
The U.S. Pat. No. 4,626,397 (Bose) issued Dec. 2, 1986 describes an improved method and apparatus, including a plurality of cooling means, for obtaining biaxially oriented high density polyethylene blown films. The improvement resides in control of physical properties in the extruded product.
The Japanese Pat. No. 84-058454/10 is worthy of mention for its disclosure of apparatus for extruding blown films of synthetic polymeric resins, which includes a mechanical means for controlling the rate of air flow (cooling means) directed at the molten tube as it leaves the extruder die.
The apparatus and the method of the present invention is useful to obtain blown films of thermoplastic resins, especially polyethylene, having predetermined physical properties, e.g. tear, impact, tensile and elongation strengths. The apparatus and method of the invention represent an improvement and advance in the art, enabling one to achieve higher production rates without loss of desired physical properties. As an example, speeds attainable using the apparatus of U.S. Pat. No. 4,626,397 are generally in the range of from 150 to 180 feet per minute. However, production speeds obtainable by use of the method and the apparatus of the present invention may reach about 220 feed per minute, without loss of physical properties in the product or any increase in breakage rates during production.